Tap changer for electric transformers



June 10, 1930. w. A. COATES ET AL 1,763,133

TAP CHANGER FOR ELECTRIC TRANSFQRMERS Filed Nov. 30, 1928 2 Sheets-Sheet l n v (q INVENTOR William A.Coates and John Fowler ATT oRNEY June 10, 1930. w, c Es ET AL 1,763,133

TAP CHANGER FOR ELECTRIC TRANSFORMERS Filed Nov. 30, 1928 2 Sheets-Sheet 2 43 Q n I 5 45 A A INVENTOR W|lliam A.Coares and John Fowler ATTQNEY Patented June 10, 1930 UNITED STATES PATENT OFFICE WILLIAM A. COATES, OF HALE, AND JOHN FOWLER, OF LEEDS, ENGLAND, ASSIGNORS TO WESTINGHOUSE ELECTRIC AND MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA TAP CHANGER FOR ELECTRIC TRANSFORMERS Application filed November 30, 1928, Serial No.

The invention relates to tap-changing apparatus for electrical transformers.

According to the invention, a transformer or a plurality of transformers are provided, each transformer having a winding provided with a plurality of tapping points, each of which is connected to an electrical switch. Mechanism is provided for successively operating said switches, and comprises a cam plate arranged for reciprocation in a straight line provided with cams which successively engage switch operating members, such as rods, to effect a predetermined change in the tap connections of the winding.

Preferably the operating rods and the cam dimensions are so related that in progressive operation either the successive switches close singly in sequence, or that alternately a single switch and a pair of adjacent switches is closed, the latter arrangement being particularly suited to the use of a tapped bridging reactor.

The cams may be provided with two p.rofiled surfaces arranged to cooperate with two sets of rollers on the switch operating rods to move the rods to close and to open the switches respectively, in which cases the switches may be held closed by suitable mechanism independently of the movement of the cam plate until the corresponding roller engages with the opening profile of the cam.-

In one arrangement of apparatus organized in accordance with the invention, and suitable for an oil-cooled transformer, the winding of which is provided with a plurality of tapping points, a rectangular opening is provided in the side of the transformer tank in which is fixed in an oil-tight manner, a panel on the outer side of which are mounted the tapping switches, one for each tapping point, connected to terminals extending throu h the panel and projecting into the trans ormer tank. The outer side of the panel is covered by a casing Which encloses 322,840, and in Great Britain December 1, 1927.

the tap-changing mechanism including the switches to be more fully described hereinafter. The casing may be filled with oil and attached to the transformer tank by an oiltight joint. This arrangement has the advantage that the usual high voltage bushings are not required to lead the tapping point conductors out of the transformer, neither is it necessary to run the oil out of the transformer in order to examine the tap-changing mechanism, as is the case with arrangements in which the tap-changing mechanism is contained in the transformer tank itself.

The switches may be of any suitable type, such as the contactor type, and are arranged to be operated by a toggle mechanism in such a way that when the switch is in the closed position, it is held closed until the toggle link is broken, after which a suitable spring forces the switch into the open position, or upon the failure of this means of o ening the switch is positively forced open y the further movement of the cam plate. Connected to the operating link of each toggle mechanism is a vertically slidableswitch operating rod and the toggle mechanism is so arranged that in order to close the switch, the rod must be moved in an upward direction, and in order to open the switch, the rod must be moved in a downward direction.

Mounted below the switches and parallel to the panel, is a spindle horizontally sup ported in suitable bearings at each end of the casing surrounding the tap-changing mechanism. The spindle is screw-threaded to receive a nut which is prevented from rotating by longitudinal guides, so that, when the spindle is rotated, the nut moves along it. Two flat cam plates are so fixed to the nut as to move therewith and are provided with two profile surfaces, one above, and the other be low the spindle. Attached to each switch operating rod is a vertical member having mounted thereon two spaced rollers, one to engage the upper cam profile and the other to engage the lower profile.

To enable the invention to be clearly understood, the particular arrangement hereinbefore referred to will now be described in greater detail with reference to the accompanying drawings, in which Fig. 1 is a front elevation of a tap-changing mechanism, for a three-phase transformer, with certain parts removed,

Fig. 2 is an end elevation thereof, partly in section,

Fig. 3 is a view on a large scale of a cam gear, and

Fig. l is a schematic diagram showing one method of connecting the tap switches thereto, that is adapted for changing tap connections without removing the load from the transformer.

Referring to the drawings, the main casing of the tap-changing mechanism is illustrated by 1, said casing being bolted to the main transformer tank, a small portion of which is indicated at 3 in Fig. 2, the transformer tank having an opening a which is closed by the back plate 5 of the casing 1 of the tap-changing mechanism.

A single one of the tap-changing switches, which are of the contactor type, is illustrated in detail onl in Fig. 2, namely at 6, while in Fig. 1 said switches are indicated by rectangles. A fixed contact member 2 of the switch is mounted on a terminal 8 which passes through an insulating bushing 9 extending through a perforation in the back plate 5 of the casing 1, so that the terminal 8 is disposed within the main transformer tank. The moving contact member 10 of the switch is carried by actuating mechanism to be hereinafter described, said mechanism being connected with a terminal 11 mounted on another insulating bushing, such as 9. The back plate 5 of the casing 1 may be replaced by a panel of insulating material, in which case the insulating bushings 9 will be unnecessary.

Fig. l shows the relation of a group of switches 6 having contact members 7 and 10 for varying the tap connections of a transformer connected between the circuit conductors a, a and b, 7).

The main operating lever 12 of the contactor switch 6 is connected by a rod 13 of insulating material to a stirrup 1 1 which is pro vided with an upper roller 15 and a lower roller 16 which rollers cooperate respectively with the upper and lower cam plates 17 and 18. These cam plates are rigidly secured to a nut 19 which operates upon a screwthreaded shaft 20, rotatably mounted at each end in bearings 21 1). The nut 19 operates between longitudinal guides 22 and is thereby prevented from rotating. The shaft has secured to it at one end, a bevel wheel 23 1 which is driven by a bevel pinion 24: secured to a universal joint 25. The upper end of the universal joint is driven by bevel gearing 2. from an electric motor 28 contained in a casing 29 which is adapted to be secured upon the top of the casing 1 of the tap-changing mechanism. The shaft 26 is also adapted to be rotated manually, being provided for this purpose with an external shaft 30 which can be turned, for instance, by means of a crank handle.

The switch actuating mechanism, which is of the toggle type, comprises a main supporting bracket 31, which carries, by means of a pivot 33, a main contact carrying arm 32, the moving contact member 10 which is itself pivoted at 3-1 to the upper end of the arm 32. The usual knuckling spring is provided in order to obtain a rolling engagement of the contact members. The main switch actuating lever 12 has a toggle portion 36 connected with the cooperating toggle link 37, which is pivoted at 36 to the arm 32. The arrange ment is such that when the contacts 7 and 10 are closed by an upward movement of the rod 13 to its full extent, the toggle members 36 and 87 will be just over the dead center position below the line passing through their outer centers 38 and 40, so that the contact will remain closed in spite of the opening spring 11, until the rod 13 is pulled downward a sufiicient extent to throw the pivot 39 to the other side of the dead center.

The profile of the upper cam plate 17 consists of a central horizontal portion -12 terminating at each end in downward oppositely sloping portions 13, the slope end length of which latter portions are sufficient to lift switch rod 13 from the open to the closed position when the shaft 20 is rotated to move me cams until the upper roller 15 rests upon the horiontal central portion 12 of the cam. The said portion 42 may be of sufficient length for the upper rollers 15 of two adjacent switch rods 13 to rest thereon at the same time as shown in Fig. 3, so that, with two adjacent switches closed. a voltage intermediate that obtained by either of the tapping points can be obtained from the transformer. The sloping surfaces 13. however, are so arranged that the first of three adjacent switch rods 13 is free to move to the upper position before the upper roller 15 of the third rod is engaged by the advancing cam.

The profile of the lower cam 18 consists of a central. horizontal portion 14 which is somewhat longer than the corresponding horizontal portion 42 of the upper cam 17 and which terminates at the ends in downwardly sloping portions 45. The parts 45 are so proportioned that they are clear of the lower rollers when the switches are open, but that when the switches are closed and the cams move so that the upper rollers leave the horizontal portion 41-2, the lowerrollc-rs are forced downwards and consequently move the operating rods in a downward direction to open the switches. The relation between the ends of the horizontal surfaces 42 and 44 of the cams may be such that an upper roller will leave the horizontal surface 42 before the lower engages a sloping portion 45, so that the upper roller can move freely downwards to allow the switch to open rapnfly under the influence of the throw-off spring 41 immediately the toggle is broken by the initial downward movement of the rod 13. v

It will be appreciated that a single screw shaft 20 may be used to operate simultaneously the respective cams for three phases of the transformer, the several cams being located as indicated in Fig. 1 symmetrically along the spindle with respect to the several sets of switches corresponding to the phases, whereby rotation of the spindle brings about simultaneously a similar change in the tap connections in each phase of the transformer.

The electric motor 28 may have associated with it in the casing 29, pilot switches controlled in accordance with the position of the mechanism, for instance, by means of cams mounted thereon, said switches being arranged in the motor circuit so as to ensure the operation of the motor through a complete cycle of operations of the tap changing mechanism. Forward and reverse limit switches may also be provided in addition to, or instead of, mechanical stops, to prevent the apparatus being operated beyond the extreme tapping positions in either direction. A tap position indicator such as illustrated at 46 in Fig. 2, may be provided, operated, for instance, through gearing from the motor 28.

The casing 1 is closed by a removable front cover 47 and may be provided with an oil level indicator 48, vents 49 and a drain cock 50.

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It will be appreciated that the description herein given of a specific arrangement is by way of example only and that many modifications may be made without departing from the scope of the invention defined by the ac companying claims.

We claim as our invention:

1. In a transformer having a winding provided with a plurality of tapping points, tap changing mechanism therefor comprising a plurality of switches for changing the connections to said tapping points. a switch operating member associated with each switch, and means for positively actuating the switch operating members in the one or the other direction, each switch comprising a movable switch arm and means for biasing the switch arm to a predetermined position, and a toggle mechanism between the switch operating member and the switch arm for locking the switch arm in a second predetermined position against the action of said biasing means until released by the switch-operating member.

2. In a transformer having a winding provided with a plurality of tapping points, tap changing mechanism therefor comprising a plurality of switches for changing the connections to said tapping points, a switch operating member associated with each switch, and means for positively actuating the switch operating members in the one or the other direction, each switch comprising a movable switch arm and means for biasing the switch arm to a switch opening position, and a toggle mechanism actuated by the switch op erating member for actuating the switch arm to, and for locking it in, a switch-closing position, said toggle mechanism upon being actu ated to a switch-opening position, permitting a rapid opening of the switch by said biasing means.

3. In a transformer having a winding provided with a plurality of tapping points, tap changing mechanism therefor comprising a plurality of switches for changing the connec-= tions to said tapping points, a switch operating member associated with each switch, and a cam for successively engaging the switch operating members, a nut for carrying said cam, a screw shaft for actuating said nut, and a longitudinal guide for said nut.

4. In a plurality of transformers each having windings provided with a plurality of tapping points, tap changing mechanisms for each transformer comprising a plurality of switches for changing the connections to said tapping points and a switch operating member associated with each switch, a cam associated with each tap changing mechanism and arranged for rectilinear reciprocation to effect a predetermined sequence of operation of the switches, and a single driving means for said cams for effecting simultaneousv and similar operation of the several tap changing mechanisms.

5. In a plurality of transformers each having windings provided with a plurality of tapping points, tap changing mechanisms for each transformer comprising a plurality of switches for changing the connections to said tapping points and a switch operating member associated with each switch, a cam associated with each tap changing mechanism and arranged for rectilinear reciprocation to effect a predetermined sequence of operation of the switches, a nut for actuating each of said cams, and a common screw shaft for actuating all of said nuts to effect simultaneous and similar operation of the several tap changing mechanisms.

6. In a transformer having a winding provided with a plurality of tapping points, tap changing mechanism therefor comprising a plurality of switches for changing the connections to said tapping points, a switch operating member associated with each switch,

upper and lower rollers connected to said switch operating member, and a cam arranged for rectilinear reciprocation for actuating the switch operating members, said cam having upper and lower profiles for engaging said upper and lower rollers to efi'ect positive opening and closing of said switches.

In testimony whereof We have hereunto subscribed our names this ninth day of November, 1928.

WVILLIAM A. COATES. JOHN FOWLER. 

